My understanding is that aspirin helps reduce clot formation by blocking an enzyme that allows platelets to stick together not truly thinning the blood. Also, that 'thinning' the blood would not help vo2, and that only training can improve your vo2 max.

On my search of PubMed http://www.ncbi.nlm.nih.gov./PubMed/ there are no published studies to date that specifically address the question you raise. Additionally, there are no studies to suggest indirectly that taking aspirin will significantly improve VO2 Max (amount of oxygen delivered to the muscle). There are some case reports of extremely high altitudes inducing Transient Ischemic Attacks (TIAs) or strokes that have symptoms that last less than 24 hours. Asprin may be of benefit here in prevention of the TIAs but these were in mountain climbers asceding Mount Everest, altitudes you will never likely be riding your bike at. I would not recommend routine aspirin usage for the suposed benefit improving VO2 max. If your doctor has you taking aspriin for other medical reasons do continue it under your physician's directions. The most important thing you can do to keep your blood from becoming too viscus at altitude (or sea level) is to keep yourself well hydrated with a sport drink whether working out in the low lands or the highlands.

If you just hydrate with plain H20 you will not be relpacing the sodium, potasium and chloride that you loose in your sweat. This will lead to hyponatremia (low serum sodium), hypokalemia (low potasium) and hypochloremia (low chloride). This can lead to muscle cramps, nausea, vomiting, abdominal cramps and in severe case of extremely low sodium seizures which can become deadly. A sports drink provides the additional benefit of providing glucose to help improve absorption of the electrolytes from the small intestins and aid in providing glucose for muscle enery usage thereby delaying depletion of muscle glycogen stores.